Boundary effect of mechanical waves of a blunt trauma can be exploited for reducing amplitude of the mechanical waves delivered to a brain tissue, using a multi-layered protective shell to increase number of boundaries inside the protective shell of a headgear as practically many as possible to a point there would not be a serious tissue injury to the brain tissue. Separately in a model of a two-layer medium panel with a first layer adjoining a second layer without a gap, it is known that there is no phase change at a boundary between the first layer and the second layer having a lower hardness than that of the first layer in reflected mechanical waves from incident mechanical waves traveling from the first layer to the second layer. Combination of both the incident and reflected mechanical waves in phase with each other temporarily increases an amplitude of the incident mechanical waves which increases an amplitude of transmitted mechanical waves in the second layer from the incident mechanical waves. If a series of the incident mechanical waves impacts the first layer, an amplitude of the reflected mechanical waves off the boundary merges with an amplitude of successive mechanical waves following a first wave of the mechanical waves coming toward the first layer. The amplitude of the successive mechanical waves following the first wave of the mechanical waves temporarily increases upon the addition of the amplitude of the reflected mechanical waves in phase with the successive mechanical waves, which increases a magnitude of an impact of the successive mechanical waves following the first wave of the mechanical waves to the second layer. If the first layer is made of a material that has a lower hardness than that of the second layer, the reflected mechanical waves off the boundary between the first and the second layers from the first wave reverse the phase and merge with the successive mechanical waves coming toward the first layer in a way the amplitude of the successive mechanical waves decreases. It results in a reduction of the magnitude of the impact of the successive mechanical waves to the second layer.
Collision between two objects is a bidirectional process involving a first object having a first kinetic energy colliding a second object having a second kinetic energy. If the first object is an inanimate object and the second object is a human head, the second object needs to reduce the kinetic energy of an impact from the first object, to transfer the kinetic energy of its own efficiently away from the second object and to lower a reflected portion of the kinetic energy of its own back to the second object in order to decrease an overall kinetic energy delivered to the second object. A method to increase the transfer of the kinetic energy of its own of the second object is to cover the second object with an A layer having a high transfer function which allows the kinetic energy from the second object to be transferred to a B layer having a lower transfer function than the A layer. The B layer is placed on an opposite side of the A layer to a side of the A layer contacting the second object. An opposite side of the B layer is placed in contact with the second layer of the two-layer medium panel. Separately, a method to reduce the reflected portion of the kinetic energy of the second object back to the second object is to use a softer material for the A layer than a bony material of the second object, and to use a softer material for the B layer than that of the A layer. In a direction toward the second object, a boundary between the B and A layers bounces off the mechanical waves residing in the B layer of both the reflected mechanical waves of the second object and a transmitted portion of the mechanical waves from the second layer of the two-layer medium panel heading toward the second object. Since the B layer is softer than the A layer, there is a phase reversal of the mechanical waves which are reflected back toward the second layer of the two-layer medium panel, thereby reducing an amplitude of the mechanical waves heading toward the second object. At a boundary between the A layer and the second object, there is another phase reversal of the mechanical waves heading toward the second object since the A layer is softer than the second object, further reducing the amplitude of the mechanical waves toward the second object.
The mechanical waves from the first object transmitted through the second layer to the B layer can be amplified or deamplified at a colliding boundary between the second layer and the B layer based on a phase of the mechanical waves inside the B layer generated from the second object. Since the phase of the mechanical waves inside the B layer generated from the second object may not be made in phase or out of phase with the phase of the mechanical waves from the first object and would not be controllable, it would be advantageous to deplete the kinetic energy of the mechanical waves inside the B layer from the second object before it reaches the second layer of the two-layer panel. It also applies to depletion of the mechanical waves from the first object coming out of the second layer heading toward the second object. One way of depletion of the kinetic energy of the mechanical waves inside the B layer is to make the B layer retain gas in a natural state and reversibly release the gas to ambient air upon collapsible compression of the B layer. Of materials transferring kinetic energy from the mechanical waves, air (gas) has by far a lowest density of molecules per area, thereby having a lowest index of transfer function as a medium for the mechanical waves. A fraction of the kinetic energy of the mechanical waves from the second object toward the first object will be released from the B layer before impacting the second layer of the two-layer medium panel and a fraction of of the kinetic energy of the mechanical waves from the first object toward the second object similarly will be released from the B layer before impacting the A layer. The depletion of the mechanical waves by releasing the gas can also be applied to the first layer of the two-layered medium panel and the A layer, except for the second layer of the two-layered medium panel. A main role of the second layer is to protect the second object against mechanical damage such as fracture of a human skull.